Trees absorb water through their roots and pass the water up the tree via transport tissue. In particular, xylem tissue, which forms part of the tree's trunk and may include tracheids, vessels, parenchyma cells, and woody fibers, transports water up the trunk and throughout the tree.
When a tree is cut down, the xylem tissues tend to close up rapidly. The tree compartmentalizes and forms a seal on the cut surface after exposure to air. This creates a problem, particularly with Christmas trees, when any measurable time passes between cutting and placing the tree in a stand with water. By the time the cut tree is exposed to water again, its ability to draw up the water is limited or entirely prevented.
The solution to this problem has historically been to make a fresh cut on the tree immediately before putting it into the tree stand. However, many consumers do not want to go through the trouble of re-cutting the tree and may not have the tools to do so. Thus, the lifespan of Christmas trees is often limited when a second cut is not done.